In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. The hot gases are then channelled towards a gas turbine which transforms the energy from the hot gases into work for powering the compressor and other devices which converts power, for example an upstream fan in a typical aircraft turbofan engine application, or a generator in power generation application.
The gas turbine stages include stationary vanes which channel the combustion gases into a corresponding row of rotor blades extending radially outwardly from a supporting rotor disk. Each rotor blade typically comprises a lower root, for coupling the blade with the turbine rotor, and a aerofoil exposed to the combustion gases channelled through the vanes. Aerofoils may be designed and manufactured hollow in order to save weight, to change its eigenfrequency or to include a cooling circuit therein.
Between the root and the hollow aerofoil, each rotor blade typically further comprises a platform, having an upper surface from which the aerofoil extends. The upper surface of the platform is therefore also exposed to the hot combustion gases channelled through the vanes. The lower surface of the platform faces the root and is instead not exposed in operation to the hot combustion gases channelled through the vanes.
Blade aerofoils and platforms at the first stages of a gas turbine are in contact with combustion gases having the highest range of temperatures, typically higher than 900° C. In such high-temperature environment, high creep strength is required for the aerofoils and the upper surfaces of the platforms.
To achieve this requirement modern alloys used for such components are very low in Cr.
On the other end, the roots and the lower surfaces of the platforms are subject to lower temperatures. In such conditions, the same low Cr alloy mentioned above can be susceptible to low temperature hot corrosion and corrosion fatigue cracking.
Potentially, a mechanical design solution could manufacture two parts to operate in different environments, but the join would always be of questionable integrity. Other solutions such as coating technologies have been readily used, but have proven to be not optimal.